Let me start by saying that I do not claim to be an expert at all, I the knowledge I have acquired over the years has been through trial and error and every once in a while I paid attention while an "old timer" talked.
Athearn is the best route starter wise, even their worst locomotives are far superior to most other brands of the past. With that being said, some other manufacturers like Bachmann and Walthers are starting to make some decent quality entry level stuff for a good price. The biggest problem with a lot of entry level power comes later on if you want to upgrade the power supply to run DCC which is digital vs a standard ac/dc power pack with directional control.
As far as the rolling stock, 10 to 15 bucks a pop is only the start! Many pieces get expensive quick. I don't really know of anywhere to get used items other than maybe Amazon, Craiglist, etc. It takes some time to build up a yards worth of cars, but it pays off long term not worrying about cheap rolling stock that derails all the time. Just make sure the couplers are mounted to the car body, not the trucks and free rolling metal wheels are far superior to plastic shaft wheelsets. Metal couplers are great, but not a necessity because they can be upgraded later on for a low cost.
I guess overall part of the reason I lost interest when I was a kid was the cheapness of a lot of the trains I had combined with my lack of knowledge of how to fix them and make them work better. Now that I can spend a little more money here and there it is a lot more satisfying hobby building on what I already have.